This invention relates to metal treating furnaces, and particularly furnaces for reheating steel slabs.
Reheat furnaces presently in commerical use are typically either pusher furnaces or walking beam furnaces. In pusher furnaces slabs are loaded one at a time into the upstream end or preheat zone of a furnace and then pushed along longitudinal, water-cooled skid rails by the introduction of successive slabs into the furnace. The slabs eventually form an unbroken line along the length of the furnace. Because the portions of the bottoms of each slab in contact with the skid rails are not directly exposed to radiant heat and to hot gases from burners which fire into the furnace to heat the slabs and because they are also cooled by the skid rails, these bottom portions develop lower temerature regions known as skid marks or dark zones. To eliminate or minimze these skid marks, pusher type reheat furnaces include a soak zone prior to the furnace outlet. After the slabs travel through the preheat zone and a heat zone, they are pushed off the skid rails onto a soak hearth in the soak zone. Here the slabs "soak" in an essentially constant temperature environment for a time sufficient to eliminate or minimize the skid marks. The slabs are then unloaded from the reheat furnace and further processed, as in a rolling mill.
Although pusher-type reheat furnaces are widely used, their soak hearths present certain drawbacks. The hearths may, for example, be less effective than desired in removing skid marks. Also, the hearths require periodic maintenance such as removal of scale and refurbishing of the hearth surface. Moreover, the presence of a soak zone occupying up to one third of the furnace length may limit furnace production rate and fuel efficiency since it requires essentially all of the slab heating to be achieved in the zones upstream of the soak zone.
In walking beam furnaces, parts are transported along the entire furnace length by mechanisms which periodically lift one or more parts off supporting rails, translate the part through a horizontal step, lower the part back onto the rails, and retract to their initial position. Since bottom portions of the parts are in contact with the rails only part of the time during heating, skid marks do not form or are greatly reduced in walking beam furnaces, and soak hearths are usually not required. However, walking beam mechanisms for slab reheat furnaces are quite complex and expensive since they must be capable of transporting slabs weighing several tons and since such walking beams must extend over the entire length of the furnace. Such complexity is also a drawback of other known devices for eliminating skid marks such as mechanisms for turning slabs over within the furnace.
Accordingly, it is an object of this invention to provide a pusher-type reheat furnace with an improved soak zone.
It is an object of the invention to provide a reheat furnace with a soak zone which eliminates or reduces skid marks without the use of a soak hearth.
It is an object of the invention to provide a reheat furnace with a simple transport mechanism and which produces high heating efficiency.